Mourinho, asked about the FFP allegations facing City, said: "It's hard for me to answer because I focus on my job".
"But what is behind, I can not say".
City won the title from United last season by a record-breaking 19-point margin with a record total of 100 points, and top the Premier League table going into the derby, with the Reds trailing by nine points.
Mourinho has spent an estimated £370 million ($480 million) on players since arriving at United in May 2016, while City manager Pep Guardiola is thought to have spent around £485 million over the same period.
Aguero, City's all-time leading scorer, was criticised heavily by Mourinho after a 0-0 draw with United at the Etihad in April 2017.
Jose Mourinho has described the quality of Manchester City's work on the pitch as "untouchable" while refusing to get embroiled in the controversy about alleged Financial Fair Play breaches made by his rivals. "But in this moment I have only to think about football, and to think about football is to think about Manchester City as a football team there".More news: Bring It Home! Florida Governor’s Race Reportedly Headed For A Recount
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'Is scoring goals in one career, is scoring goals [in] like the Spanish league'.
The pair's relationship has reportedly been on the rocks this season, perhaps playing some part in United's inconsistent form.
'There are little things in his game that I'm not a big fan of.
When asked what he didn't like about Aguero's game, Mourinho replied: 'I keep for me'.
A rather hard run of fixtures for the Red Devils will conclude with an away match against Manchester City in the Premier League.
Mourinho has praised United's ability to fight back from losing positions, but suggested he would like to see them score the first goal more often. "We have of course one more session tomorrow (Saturday), if the answer is positive he is ready", Mourinho added.